It's not suitable to take a sleep in it (5)
I believe the answer is:
inapt
'it's not suitable' is the definition.
(I've seen this in another clue)
'a sleep in it' is the wordplay.
'a sleep' becomes 'nap' (a nap is a short sleep).
'in' is an insertion indicator.
'nap' placed into 'it' is 'INAPT'.
'to take' is the link.
(Other definitions for inapt that I've seen before include "not seemly" , "Unsuited" , "Improper" , "Not fit, unsuitable" , "Free from deceit" .)